Avoid vue warning in dev mode (go-gitea#29188)

`vue` currently outputs a warning for this undefined variable during
development, which is apparently caused by a bug in `vue-cli`.
Workaround by setting this variable.

Ref: vuejs/vue-cli#7443
Ref: https://stackoverflow.com/a/77765007/808699
(cherry picked from commit e9a1ffb)

webpack.config.js

@@ -176,6 +176,7 @@ export default {
     new DefinePlugin({
       __VUE_OPTIONS_API__: true, // at the moment, many Vue components still use the Vue Options API
       __VUE_PROD_DEVTOOLS__: false, // do not enable devtools support in production
+      __VUE_PROD_HYDRATION_MISMATCH_DETAILS__: false, // https://github.com/vuejs/vue-cli/pull/7443
     }),
     new VueLoaderPlugin(),
     new MiniCssExtractPlugin({